Argon is used in the metallurgical industry, particularly in argon-oxygen degassing of stainless and specialty steels and in the cutting and welding of various metals. Plasma jet torches, utilizing an argon mixture heated to temperatures in excess of 10,000 degrees K, are used for cutting operations and for coating metals with refractory materials. More recently argon has become an important ingredient in the electronics industry as a carrier, purge, or blanketing gas to exclude air from certain fabrication processes, especially in the growing of crystals, ion milling, and other etching processes.
The production of argon is an important economic factor in the industrial gas industry. Generally argon is a by-product of cryogenic air separation. However, a number of additional processing steps are necessary to produce a required purity of argon. One of the critical purity requirements is the concentration of contained nitrogen. Many applications of argon demand that it be essentially free of nitrogen.
The use of structured packing in cryogenic distillation columns has presented an opportunity to take advantage of the packing's characteristics of good mass transfer accompanied by low pressure drop (e.g., see U.S. Pat. No. 4,296,050 to Meier). The addition of a large number of theoretical trays in the low pressure column of a cryogenic air separation plant, without incurring the effects of an accompanying large pressure drop, by the use of structured packing presents a significant economic improvement in the production of argon.
In the past, the production of high purity argon involved a number of processing steps to produce a crude argon stream which was then upgraded in a refinery. Argon processing starts with the low pressure column of a cryogenic air separation plant. A low grade argon stream is withdrawn from an intermediate point in the low pressure column. The low grade argon stream is then fed into an argon column where it is separated into an overhead crude argon stream containing about 97.5 percent argon and a bottom stream which is returned to the low pressure column. The overhead stream also typically contains about 1.5 percent oxygen and about 1.0 percent nitrogen.
The crude argon stream from the top of the argon column is then warmed to about ambient temperature, at which time hydrogen is added and the mixture compressed and sent to a Deoxo catalytic furnace where the oxygen is removed. The combusted argon is cooled, dried and then further cooled to essentially liquefaction temperature. The cold argon stream is then sent to the refinery column where the excess hydrogen and remaining nitrogen are removed. Normal production provides an argon product stream containing less than 5 ppm nitrogen or oxygen.
German Patent 1 048 936, describes a means for reducing the nitrogen content of the feed to an argon column. The suggested process increases the number of trays used in the section of the low pressure column, between a feed from the argon condenser and the point where the argon column feed is withdrawn. The use of additional trays in the low pressure column, for the purpose of reducing the nitrogen content of the feed to the argon column, imposes a pressure drop penalty which increases the air compressor discharge pressure and therefore the energy requirements. Further, the increase in pressure level reduces relative volatility within the columns, resulting in a lowering of argon recovery.
In U.S. Pat. No. 5,133,790, Jul. 28, 1992, to Bianchi et al., (the disclosure of which is incorporated herein by reference), the use of structured packing is suggested to increase the number of equilibrium stages in the low pressure column between the feed from the argon condenser and the point where the argon column feed is withdrawn. The additional rectification in the lower pressure column is provided by the incorporation of structured packing rather than by trays. This reduces the nitrogen concentration substantially while maintaining the argon concentration at or near its maximum, enabling the production of nitrogen-free argon directly. The use of structured packing, rather than trays, avoids the energy penalty and reduced argon recovery.
Full scale testing of the system proposed by Bianchi et al., (which utilizes structured packing throughout the low pressure column), revealed that it is difficult to achieve low nitrogen levels in the argon column feed. Attempts were made to achieve low nitrogen levels using a single bed of packing between the feed from the argon condenser and the point where the argon column feed is withdrawn. The performance was not satisfactory.
It is an object of this invention to provide an improved argon production system which employs a low pressure distillation column with structured packing.
It is another object of this invention to provide an improved argon production system wherein the feed from a low pressure column to an argon column is largely nitrogen free.